msm: mdss: Fix race condition during mdp dump

In mdp debugfs, pipe, mixer, buffers and ctl information
is dumped. While dumping pipe buffer information using
pipe's buffer list, acquire list lock mutex. This is
done to prevent crash if at the same time buffer is freed
in commit thread.

Change-Id: Ibd26cc1484fb08ae7d0852f405100f6bbfa707de
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>